The Exchange 2000 release notes contain the following information:
Installing Active Directory Connector in a Child Domain
If you want to install Active Directory Connector (ADC) into a child domain, you must first extend the Active Directory schema by running the Active Directory Connector setup from a command line in the domain where the schema master is located and using the /schemaonly flag. The root domain is used by default. Then, after information is replicated to the child domain, you can run the Active Directory Connector setup in the child domain using a user account from the child domain.
To run setup for ADC, the account you are logged on as must be a member of the Enterprise Administrator group. However, if the root domain is a mixed mode domain, you cannot add other groups or user accounts from other domains to the Enterprise Administrator group.
If the root domain is in native mode and the child domain is in mixed mode, you can add an account from the child domain, but the group membership is not replicated to the mixed mode child domain.
To successfully run setup while logged on as an account in the child domain, both the root and child domain must be in native mode and your account must be a member of the Enterprise Administrator group.
When the root domain is in mixed mode, the Enterprise Administrator group type is "Security Group - Global". By definition, global groups cannot contain users or groups from other domains. In addition, you cannot add other groups in the same domain to a global group when the domain is in mixed mode.
When you change the domain to native mode, the Enterprise Administrator group type becomes "Security Group - Universal". Universal groups can contain other groups (global or universal) and user accounts from other domains.